580 HANDBOOK OF PHYSIOLOGY. 



si on of semen takes place, over which the mind has little or no control, 

 and which, in cases of paraplegia, may be unfelt. 



(d.) The Erection of the Penis centre. This centre is also situated 

 in the lumbar region. It is excited to action by the sensory nerves of 

 the penis. Efferent impulses produce dilatation of the vessels of the penis, 

 which also appears to be in part the result of a reflex contraction of the 

 muscles by which the veins returning the blood from the penis are com- 

 pressed. 



(e.) Parturition centre. The centre for the expulsion of the con- 

 tents of the uterus in parturition is situated in the lumbar spinal cord 

 rather higher up than the other centres already enumerated. The 

 stimulation of the interior of the uterus by its contents may, under 

 certain conditions, excite the centre to send out impulses which produce 

 a contraction of the uterine walls and expulsion of the contents of the 

 cavity. The centre is independent of the will since delivery can take 

 place in paraplegic women, and also while a patient is under the influ- 

 ence of chloroform. Again, as in the cases of defecation and micturi- 

 tion, the abdominal muscles assist; their action being for the most 

 part reflex and involuntary. 



(/. ) The Centre for the Movements of Lymphatic Hearts of Frog. 

 Volkmann has shown that the rhythmical movements of the anterior pair 

 of lymphatic hearts in the frog depend upon nervous influence derived 

 from the portion of spinal cord corresponding to the third vertebra, and 

 those of the posterior pair on influence supplied by the portion of cord 

 opposite the eighth vertebra. The movements of the heart continue, 

 though the whole of the cord, except the above portions, be destroyed ; but 

 on the instant of destroying either of these portions, though all the rest of 

 the cord be untouched, the movements of the corresponding hearts cease. 



((/.) The Centre for the Tone of Muscles. The influence of the spinal 

 cord on the sphincter ani and sphincter urethras has been already men- 

 tioned (see above). It maintains these muscles in permanent contrac- 

 tion. The condition of these sphincters, however, is not altogether 

 exceptional. It is the same in kind, though it exceeds in degree that 

 condition of muscles which has been called tone, or passive contraction; 

 a state in which they always when not active appear to be during health, 

 and in which, though called inactive, they are in slight contraction, and 

 certainly are not relaxed, as they are soon after death, or when the spinal 

 cord is destroyed. This tone of all the muscles of the trunk and limbs 

 depends on the spinal cord, just as the contraction of the sphincters 

 does. If an animal be killed by injury or removal of the brain, the 

 muscles retain their tone; but if the spinal cord be destroyed, the 

 sphincter ani relaxes, and all the muscles feel loose, flabby, and atonic, 

 remaining so till rigor mortis commences. 



